The present invention relates to a projection type image display device, which enlarges an image displayed on an image source by means of a projecting lens to display it on a screen, and in particular to a projection type image display device, an optical system therefor and a projecting lens therefor, which are suitable for intending to reduce color fluctuations in the image on the screen.
The present invention is useful for reducing color fluctuations in the image, in the case where it is intended to shorten the distance from the image source to the screen to reduce the size of the display device.
Recently the projection type television, by which images displayed on a small image sources such as projection type cathode-ray tubes are enlarged by means of projecting lenses to be projected on a screen, is more and more widely utilized for home use and business use, because improvement in the image quality is remarkable and it is possible to enjoy images on a great screen with impressive presence, aided by the fact that semiconductor large capacity memories have been developed and that it has become easier to effect digital control of the dynamic convergence of the cathode-ray tube, etc.
In the projection type television, in the case where a projection type cathode-ray tube is used as an image source, it is usual heretofore that a cathode-ray tube and a projecting lens are combined for each of the three primitive colors, red, green and blue, and an image of three primitive colors is synthesized on a screen, as disclosed e.g. in JP-A-Sho 61-95689, in order to increase satisfactorily the brightness of the image on the screen.
According to the prior art technique described above, the cathode-ray tubes of the three primitive colors are juxtaposed in the direction from the left to the right of the screen. Usually the green cathode-ray tube is located at the center and the red cathode-ray tube and the blue one are located on the left and the right side thereof, respectively, so that the optical axes of the different colors are intersected with each other at one point in the neighborhood of the screen. The projection directions for the images of the different colors, red, green and blue, to the screen are different from each other and the tendences of the distributions of the brightness on the screen are different for the different colors. Therefore, e.g. when a white image is projected on the whole surface of the screen, fluctuations in the color are produced, i.e. the image is locally reddish or bluish. These fluctuations in the color will be explained below quantitatively.
FIG. 1 is a scheme illustrating the prior art projecting optical system developed in the horizontal direction, in which 1R, 1G and 1B represent a red, a green and a blue projection type cathode-ray tube, respectively; 2R, 2G and 2B are projecting lenses for the projection type cathode-ray tubes 1R, 1G and 1B, respectively; 8 is a screen; 8C is the center of the screen; 8R is the right end of the screen; 8L is the left end of the screen; 9R, 9G and 9B are a red, a green and a blue light beam, respectively; and 3R, 3G and 3B are optical axes of the projecting lenses 2R, 2G and 2B, respectively.
FIG. 2 is a front view of the screen 8, on which a coordinate system is so defined that the origin is positioned at the screen center 8C; the positive direction of the x-axis is directed towards the right in the horizontal direction; and the positive direction of the y-axis is directed upward in the vertical direction. A point P(x, y) is an arbitrary point on the screen 8.
Further, in FIGS. 1 and 2, W.sub.H and W.sub.V represent the width and the height of the screen, respectively; D indicates the distance between the exit pupils (not shown in the figure) of the projecting lenses 2R, 2G and 2B and the screen center 8C; .theta. is an angle offset, with which the optical axes of the projecting lenses are converged; and 10R, 10G and 10B are optical axes of the light beams from the cathode-ray tubes 1R, 1G and 1B to the point P on the screen 8, respectively.
The illuminance at the point P(x, y) on the illuminated side of the screen 8 due to the light beams from the cathode-ray tubes 1R, 1G and 1B to the screen 8, when a white image is projected on the whole surface of the screen, will be calculated.
For the sake of simplicity it is supposed that the cathode-ray tubes 1R, 1G and 1B are perfect diffusing surface light sources, which are infinitely distant from the projecting lenses 2R, 2G and 2B, respectively. That is, it is supposed that all the light beams from the cathode-ray tubes 1R, 1G and 1B to the point P on the screen 8 enter the projecting lenses 2R, 2G and 2B as parallel light beams having a uniform brightness, respectively.
Further it is supposed that the light beams entering the projecting lenses 2R, 2G and 2B from the cathode-ray tubes 1R, 1G and 1B are emitted by the projecting lenses 2R, 2G and 2B through the center of the exit pupil thereof, respectively, without any loss.
Here the angles formed by the optical axes 10R, 10G and 10B from the cathode-ray tubes 1R, 1G and 1B to the point P on the screen 8 and the optical axes 3R, 3G and 3B are denoted by .omega..sub.LR, .omega..sub.LG and .omega..sub.LB (hereinbelow called image angles of the point P), respectively; the angles formed by the optical axes 10R, 10G and 10B and the normal to the screen 8 by .omega..sub.SR, .omega..sub.SG and .omega..sub.SB (hereinbelow called screen incident angles), respectively; the distances between the exit pupils of the projecting lenses 2R, 2G and 2B and the point P by D.sub.R, D.sub.G and D.sub.B, respectively; and the illuminances for red, green and blue at the point P by E.sub.R, E.sub.G and E.sub.B, respectively; the illuminances for red, green and blue at the center 8C of the screen by E.sub.RO, E.sub.GO and E.sub.BO.
Further, in the following calculation formulas, a suffix i indicates R, G and B, which represent the red, the green and the blue, respectively.
Under the suppositions described above the illuminance E.sub.i for the different colors at the point P is proportional to the area of the entrance pupil of the projecting lenses 2R, 2G and 2B, respectively. This area of the entrance pupil is proportional to cos.omega..sub.Li, when the image angle at the point P is .omega..sub.Li. Further, in the case where each of the projecting lenses is composed of a plurality of lens elements, "vignetting" of the light is often produced due to the fact that the aperture of each of the lens elements is finite, and if the aperture efficiency at this time is supposed to be represented by a function of the image angle .omega..sub.Li at the point P, i.e. V(.omega..sub.Li), the illuminance E.sub.i at the point P is proportional to V(.omega..sub.Li) Still further, the illuminance E.sub.i at the point P is inversely proportional to the square of the distance D.sub.i between the exit pupil of the projecting lens 2R, 2G or 2B and the point P according to the inverse distance square law and proportional to cos.omega..sub.Si, .omega..sub.Si representing the screen incident angle, according to the law of cosinus of incident angle.
Consequently the illuminance E.sub.i at the point P can be expressed by; ##EQU1## where i=R, G, B, ##EQU2## Further, according to the cosinus theorem, cos.omega..sub.Li and cos.omega..sub.Si can be given by ##EQU3## respectively.
At this time, if i =G, i.e. for the green projected light, since EQU cos.omega..sub.Li =cos.omega..sub.Si =D/D.sub.i
using Eqs. (3), (4) and (5), Eq. (1) can be transformed into: EQU E.sub.G =E.sub.GO V(.omega..sub.LG)cos.sup.4 .omega..sub.LG.
Thus it can be understood that the illuminance at the point P on the screen 8 is proportional to the fourth power of casinus of the image angle .omega..sub.LG. This is no more than a formula according to the so-called cosinus fourth power law.
A concrete example of the illuminance at the point P, where the illuminance at the screen center 8C, E.sub.i0 =1, will be indicated below (hereinbelow called relative illuminance).
FIG. 3 shows the distribution of the illuminance in the horizontal direction passing through the center on the incident surface of the screen 8, in which the abscissa represents the position in the horizontal direction and the ordinate the relative illuminance, when all the illuminances for red, green and blue at the screen center 8C are equal to 1, the broken line indicating the distribution of the illuminance for red; the dotted line that for green; the full line that for blue. For this calculation it is supposed that the size of the screen 8 is 40" measured along the diagonal; WH=812 mm; W.sub.V =610 mm; D=900 mm; and .theta.=8.degree.. Further the aperture efficiency V(.omega..sub.Li) of the projecting lens is approximated by; EQU V(.omega..sub.Li).apprxeq.cos.sup.8 .omega..sub.Li.
From FIG. 3 it can be understood that, contrarily to the fact that the distribution of the illuminance for green is symmetric with respect to the center of the screen, the distribution of the illuminance for red is deviated towards the left on the screen and that for blue is deviated towards the right on the screen. This deviation increases with the increasing optical axis convergence angle offset .theta..
As the result, with reference to the white color in the neighborhood of the center of the screen, from the left upper corner to the left lower corner of the screen, since the relative illuminance for red is higher and that for blue is lower than that for green, the color temperature is low and it seems to be reddish or yellowish. On the contrary, from the right upper corner to the right lower corner of the screen, since the relative illuminance for red is lower and that for blue is higher than that for green, the color temperature is high and it seems to be bluish or cyanish. For this reason the observer recognizes the presence of fluctuations in color.
On the other hand, on the manner how the fluctuations in color are seen, there is known in the visual sensation psychology a perception phenomenon called "color contrast effect". This is a phenomenon that, when a region of a certain color (test field) is enclosed by a region of another color (inducing field), the color in the test field is seen, as if it were changed in another color, in which the complement of the color in the inducing field is added to the color itself in the test field.
As described above, in the prior art projection type optical system, when a white image is projected on the whole surface of the screen, a region, which is more reddish or yellowish, and a region, which is more bluish or cyanish than the neighborhood of the center of the screen, are produced. Since these regions are in a relation that the colors therein are complementary to each other, the fluctuations in the color are seen, emphasized by the color contrast phenomenon. These fluctuations in the color is in a level, which is so high that the observer can recognize them, not only when a white image is projected on the whole surface of the screen but also when a general image of the television broadcast is projected. Therefore there was a problem that the quality of the image was impaired significantly.
In particular, in a projection type image display device, for which it is required to reduce the size of the space for mounting the device as far as possible, such as a rear projection type television for home use, it is necessary to shorten the projection distance from the projecting lens to the screen in the projecting optical system, in order to reduce the size of the casing for the device. In this case, since the optical axis (convergence) angle offset formed by the optical axes of the projecting lenses for the different colors increases, the fluctuations in the color are increased further. For this reason there was a problem that it was prevented to reduce the size.
The object of the present invention is to provide a compact projection type image display device, an optical system therefor as well as a projecting lens used therein capable of solving the problems of the prior art technique described above and obtaining images of high quality, while realizing the compatibility of shortening the projection distance from the projecting lens to the screen in the projecting optical system and reducing the fluctuation in the color on the screen.